In the field of the clinical and biochemical analysis, there is an increasing interest in the use of microarrays.
A microarray comprises a substrate made of a suitable material, e.g. glass, polymer coated glass, plastics, nitrocellulose, and the like, on which a plurality of spots of one or more biological fluids, e.g. proteins, cells, DNA fragments, and similar, are fixed. The size of the substrate is, typically, of about some inches, e.g. one inch by three inches, and the spots fixed thereon usually ranging from about 50 μm to about 300 μm in diameter.
Microarrays are important tools for the biological research, in particular genomics, proteomics, and cell analysis, and they are used for the diagnosis and prevention of a lot of diseases. For example, deoxyribonucleic acid (DNA) and protein microarrays, also called “biochip microarrays” have accelerated the process of understanding gene and protein functions in living organisms.
Microarray fabrication is inherently a deposition process of a biological fluid onto a substrate. Very small quantity (nanoliters or picoliters) of one or more biological fluids are deposited onto the substrate in the shape of spots.
For depositing spots, two types of devices are known in the art: “contact printing” devices, e.g. pin printing and microstamping devices, and “non-contact printing” devices, e.g. inkjet-like printing devices, both thermal and piezoelectric.
As it is known, a thermal inkjet printing device comprises a system for feeding a sheet of paper on which an image is to be printed, a carriage driven by a motor in a direction perpendicular to the sheet feeding direction, printing means, typically one or more printheads carried by the carriage and in fluid communication with respective ink reservoirs, and electric power supply means. In particular, each printhead comprises at least one nozzle, typically an array of nozzles, a firing chamber and a heating element.
In use, the electric power supply means supplies energy to the printhead as a pulse electric signal. During each pulse, the heating element warms up of several hundred degrees in few microseconds. A small quantity of ink passes from one of the ink reservoirs to the firing chamber, where it comes into contact with the heating element. Following the contact with the heating element, the ink quickly warms up, thereby generating a vapor bubble inside the firing chamber. The resulting vapor bubble expansion, and the subsequent collapse thereof, cause an ink drop to be ejected through the at least one nozzle of the printhead.
Typically, the thermal inkjet printing devices allow to eject ink drops having a volume ranging from 200 pl to 2 pl, or smaller.
Volume and ejection speed of a drop may vary according to the energy E supplied to the printhead. The threshold energy Eth depends on the geometrical characteristics of the printhead, as well as on the thermodynamic and fluidodynamic properties (e.g. boiling point, surface tension, density and viscosity) of the used ink.
It has been experimentally noticed, see for example the U.S. Pat. No. 5,767,872, that for values of the energy E below the threshold energy Eth the one or more nozzles of the printhead do not eject any drop. This is because the heating element does not reach a temperature high enough to cause the generation of the vapor bubble. Above the threshold energy Eth, there is a so called “transitional” or “drop instability” zone, where the drop volume increases with increasing the energy E supplied to the printhead. Above the transitional or drop instability zone there is a so called “drop stability” zone, where the drop volume remains substantially constant with increasing the energy E supplied to the printhead.
In order to guarantee the uniformity of the printed images, the drops ejected through the nozzles should preferably have a constant volume. It is thus advisable the printhead to be operated in the drop stability zone.
Therefore, it is defined as “threshold energy” Eth the minimum energy suitable for causing drop ejection through at least one nozzle of a thermal printhead.
Moreover, in the following description, the expression “stable drop” will designate a drop having a constant volume with respect to the energy E supplied to the printhead, whereas the expression “instable drop” will designate a drop having a variable volume with respect to the energy E supplied to the printhead.
However, it has been experimentally observed that if the energy E increases too much with respect to the threshold energy Eth, although the drop remains stable, the printhead is subjected to a premature ageing, and consequently its lifetime decreases. This is believed to be caused by an excessive warming of the heating element and by the subsequent build-up of ink residues onto the surface thereof. This phenomenon is known with the term of “Kogation”.
In fact, the high temperatures reached by the heating element (about 350° C.) cause a degradation of the additives, typically the dyes, present in the ink. Additives are not soluble in the ink, whereby they deposit onto the surface of the heating element, thus forming a proper insulating layer. This insulating layer decreases the thermal efficiency of the heating element and, at the worst, causes the rupture of the printhead.
To obviate the phenomenon of Kogation, and thus to increase the lifetime of a printhead, it is known to supply the printhead with an energy greater than the threshold energy, thereby allowing the ejection of stable drops. However, the energy supplied to the printhead should not be so great to cause the build-up of the insulating layer onto the surface of the heating element. This is disclosed, for example, in the U.S. Pat. Nos. 6,302,507 and 6,315,381, which describe a system and a method for controlling the energy applied to a thermal inkjet printhead assembly.
U.S. Pat. No. 6,575,548 describes a printing system and protocol for providing efficient control of energy characteristics of an inkjet printhead. The printing system includes a controller, a power supply and a printhead assembly having a memory device and a distributive processor integrated with an ink driver. The distributive processor maintains energy characteristics of the printhead assembly within preprogrammed acceptable boundaries. More specifically, the energy supplied to the printhead is approximately 20% over the threshold energy defined above.
U.S. Pat. No. 7,281,783 describes a fluid ejection device comprising a resistor, a chamber, a first fluid channel and a second fluid channel each communicated with the chamber. To ensure a stable operation, the resistor is supplied with an energy approximately 25 to 50% over the minimum energy or threshold energy.
Thanks to the small amount of liquid dispersed per unit of area and the low production costs of the thermal inkjet printheads, the thermal inkjet printing technique could be employed in the field of the clinical and biomedical analysis. In this case, the ink would be replaced by one or more suitable biological fluids.
U.S. Pat. No. 6,935,727 describes a method for depositing fluids, typically fluids containing a biopolymer or precursor thereof, onto a substrate surface by using a pulse jet printhead assembly. In use, the firing chamber of the printhead assembly is loaded with a volume of fluid that includes a biopolymer or precursor thereof. The loaded printhead assembly is then placed in opposing relation to a surface of a substrate and actuated to deposit a volume of fluid on the substrate.
US patent no. 2003/0027219 describes a method for efficiently depositing small quantities of a protein containing fluid onto the surface of a substrate by using thermal inkjet printing apparatus. The disclosed depositing process does not substantially modulate the protein activity/functionality of the deposited fluid. In practicing the method, a small volume of fluid containing the protein(s) of interest is front loaded into a thermal inkjet device. Next, a small quantity of the front loaded fluid is expelled onto the surface of the substrate.
However, the above cited U.S. Pat. No. 6,935,727 and 2003/0027219 do not face the problem of the degradation of a thermal inkjet printhead.